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Elephants have them. Pigs have it. Narwhals and water deer have them. Teeth are among the most dramatic examples of mammalian teeth: protruding teeth that are constantly growing, used for fighting, foraging, and even dating.
So, over the broad sweep of geological history, why have such useful teeth only appeared among mammals and no other surviving animal groups? according to Study published Wednesday In the journal Proceedings of the Royal Society B, It takes two important adaptations in teeth to make a tooth – and the evolutionary pathway first appeared millions of years before the first true mammals.
About 255 million years ago, A family of mammal relatives called Dicynodonts – tusked, turtle-billed herbivores ranging in size from gopher-sized burrows to six-ton giants – roamed the forests of the supercontinent Pangea. Few lineages survived the devastating Permian extinction period, in which more than 90 percent of Earth’s species died out, before being replaced by herbivorous dinosaurs.
“They were really successful animals,” said Megan Whitney, a Harvard University paleontologist and lead author of the study. “They’re so abundant in South Africa that you get really tired of seeing them at some of these sites. You’re going to look at a field and there will be skulls of these animals everywhere.”
To understand how these animals develop their teeth, Dr. Whitney and colleagues collected bone samples from 10 species of dicynodont, among them the small, big-eyed Diictodon and the tank-like species. Lystrosaurus. They looked at how their dogs were attached to the jaw, whether, like many reptiles, they regularly regenerated lost teeth, and for indications that their teeth were constantly growing.
Many mammalian families have developed long, saber-toothed teeth or ever-growing incisors for gnawing. A few early dicynodonts also had a pair of long canine teeth protruding from their beaks. However, these teeth, like most animal teeth, consist of a substance called dentin, which is covered with a hard, thin layer of enamel. Dr. The teeth have no enamel and grow steadily, even as the relatively softer dentin wears, Whitney said.
Examining the Dicynodont skulls, the team found that in the middle of the group’s evolution, a change occurred: the appearance of soft tissue appendages that support teeth, similar to ligaments found in modern mammals. And like modern mammals, dicynodonts didn’t constantly change their teeth.
Both of these shifts laid the foundation for the development of an ever-growing, well-supported tooth – a tooth. Later, Dr. Late dicynodonts developed teeth in at least two different lineages, and possibly more, Whitney said.
This evolutionary path resembles another group of tusked animals: elephants. Dr. The first elephant relatives had enlarged canines covered with enamel, Whitney said. Later members of the family made the enamel into a thin band on one side of the tooth, like a rodent incisor, allowing the tooth to grow continuously. In the end, they completely removed the enamel.
Dr. “If you unlock the evolution of reduced tooth replacement and soft tissue attachments, you provide the tools for the tooth to evolve,” Whitney said. “Once you have a group with both conditions, you can go a long time with animals playing with different combinations of teeth and start to see these independent developments of the teeth.”
The reason teeth are currently restricted to modern mammals lies in a specialized dental array that mammals inherit from the larger synapse family that includes mammalian ancestors such as dicynodonts.
Even with these prerequisites, Dr. An adaptation like teeth is not inevitable, Whitney said. But there are and many groups of mammals – elephants, whales, deer, pigs and walruses – have found uses for them.
Dr. “Mammals are stuck in our teeth, unlike something like a shark, which is a kind of terror carrier,” Whitney said. “So if you’re only replacing your tooth once, an ever-growing tooth is pretty shiny.”
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